How many people have bowled back to back perfect games?

With the advent of high-tech Bowling balls and forgiving lane oil patterns, tens of thousands of perfect 300 games are bowled every year. While a good number of them are bowled by people who have already done so at least once, there are still several thousand first-time 300s bowled each year.

In fact, as of April 2010, 15 people have officially bowled a 900 series -- which is three 300 games back-to-back in the same league or tournament session. (The actual number of 900s bowled is over 20, but only 15 have been recognized by the USBC.)

Fun fact: in duckpin and candlepin bowling, two variations of the sport, no 300s have ever been rolled.